Electron discharge device



G. A. BURDICK ELECTRON DISCHARGE DEVICE Feb. 7, 1961 2 Sheets-Sheet 1 Filed Nov. 10, 1958 2 V0 w w M, 3 5 M E BEAM CURRENT (a. a.

I NVENTOR Glen 4 Bard/ck fazferf 5W ATTORNEY Feb. 7, 1961 e. A. BURDICK 2,971,118

ELECTRON DISCHARGE DEVICE Filed Nov. 10, 1958 2 Sheets-Sheet 2 Fig.5 E9] 3c; I

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7 I #JTTT INVENTOR 67m 4. fiard/ck BY TM 5 W ATTORNEY U iwd Sta es iat i jl 53? 2,971,118 ELECTRON DISCHARGE DEVICE Glen A. Burdick, Waterloo, N. Y assignor, liymesne assignments to Sylvania Electric Products In'c., Wilf mington, D'eL, a corporation of Delaware Filed Nov. to, 1958, Ser. No. 773,113 's'c'laims. (cl. 315-166 I This invention gener lly relates to electron discharge devices and more particularly to cathode ray tubes of the type employing electrostatic means for focusing the electron beam used to excite the image screen in the tube.

The majority of cathode ray tubes presently produced for image display purposes in electronic equipment such as television receivers conventionally use a uni-potential focusing lens system in the electron gun to provide final focusing of the electron beam. This type of lens is usually formed by an array of three aligned cylindrical elec trodes mounted at the end of a tetrode electron gun lo= cated nearest the image screen. The first and third elec= trodes in a uni-potential focusing system, which are nor; mally referred to as lens cups, are operated at the same potential, i.e., the anode or image screen potential. This anode Voltage may range from 12 to 24 k'v}, depending upon the application. The intermediate second electrode or lens ring normally is operated at a potential which Varies in accordance with the tube application from 300 volts negative to 500 volts positive. A lens of this type is net convergent so as to cause the diverging electron beam entering the lens from the prefoc'usin'g lens to converge or focusat the screen. I

Although a uni-potential lens of the type described above performs satisfactorily, it requires a moderately long gun mount and thus a long tube neck, e.g., inches. A cathode ray tube having a long neck is undesirable in light of the present trend in the television industry toward a reduction in axial tube length. At the present state of the art, a unipotential lens type 'gun can not be shortened beyond a given length Without poor electron beam focusing which is manifested as poor picture resolution. This disadvantage seems, at present, to be inherent in a uni potential lens system due to its high magnification char act'eristics. Also, this type of lens has a high degree of spherical aberration, which causes distortion of the beam spot on the screen when the magnetic deflection fields are" allowed to penetrate the lens electrode position. In order to make a minimum axial length tube, it is desir able to move the electron gun as far forward into the mag netic deflection field as possible, but this, as explained, in creases aberration in the uni-potential electrostatic focus gun.

Another type of electron gun structure which has been proposed uses a bi-potentia'l electrostatic collimating lens for focusing. This mount employs four spaced grids G G G and G respectively, arrayed from the cathode toward the screen. Voltages on the grids in this type of electron gun are so arranged that G; may be usedfor conventional electron control purposes, while Gs may serve, in one aspect, to provide an immersione1etr s= static field for forming the electron beam crossover point. G is shaped with a restricted openingterminating in a surface defining a spherical opening to form a collimating convergent electrostatic field when operated at a low voltage relative to the potential applied to the open en'decl cylindrical shaped G 2,971,118 Patented liebr 7,1251

Under normal conditions, this collimating bi-potential focus mount has three lenses or lens fields. The first or immersion lens extends from G to the cathode and aids in the production of the beam and formation of a crossover point. The second lens is positioned between G and G and serves to some degree as an isolating field and as a so=called prefocusing lens.- The third or collimating lens formedby G and G provides final focusing of the beam. In this type of system, the limiting aperture in G isolates the first lens from the second and the restricted opening and general configuration of 6;, isolates the second lens from the third lens.

A- gun mount of the bi-potential type described above has a number of disadvantages. Due to the complex configuration of the parts, it is costly to produce and does not lend itself to automatic production techniques.

Also, there is a tendency in this type of structure to cause beam de-focusing when there are variations in television receiver supplied voltages which result from incoming power line voltage fluctuations. Further, in a collimating lens, the action of thesecond focusing field operating in conjunction with the G configuration produces a lens having a large amount of spherical aberation. When an electron beam passes through such a lens, poor focusing characteristics are experienced at wide deflection angles or at the edges of the screen even when the beam emerges from the center of the lens. This defocusing phenome non is magnified when the conventional centering inag net normally positioned on the tube neck must be used. In other words, a lens having suflicient spherical aberration to cause undesirable beam spot distortion or defocusing' when the beam emerges from the center of the focusing lens causes additional spot distortion when the beam is forced, by a centering magnet, to emerge from a position on the lens which is not located at the center.

Accordingly, an object of this invention is to reduce the aforementioned disadvantages in an ultra-short length cathode ray tube capable of reproducing satisfactory images.

A further object is to facilitate the utilization of low cost electrodes in a short electron gun mount which may easily be assembled and aligned by automatic production techniques.

Other objects of the invention are to reduce the spherical aberration of the electrostatic focus type short oath ode ray tube mount so that it can be moved into the deflection region without causing defocusing or beam spot distortion; to reduce the magnification of a lens produced by a short focusing system so that proper beam focusing at the screen may be achieved; to utilize voltages for the electrodes in the focusing system which are readily available in a television receiver; and to produce a lens which is relatively insensitive, from a focusing viewpoint, to fluctuations in line voltage. v

The foregoing objects are achieved in one aspect of the invention by the provision of a cathode ray tube having a relatively short electron gun which employs an electrostatic focusing lens system of a type wherein the object distance is relatively short and the net potential in the convergent region of the lens can be controlled without unduly afiecting the immersion lens. In one embodiment of the invention, the focusing system is formed by whli ch is adapted for use in the cathode ray tube shown in 1g.

Fig. 3 shows the electrostatic field configuration of the electrostatic lens system produced by the gun shown in Fig. 2 and its effect on the electron beam;

Fig. 4 is a graphical representation of certain characteristics of the gun structure shown in Fig. 2;

Fig. 5 illustrates another gun structure of the discring yp Fig. 6 illustrates another gun structure of the disc yp Fig. 7 shows a Figure 2 type of gun modified primarily in the cathode and first grid regions; and

Fig. 8 shows another ring type gun structure employing a larger number of electrodes. 1

Referring to Fig. 1, a cathode ray tube 11 is shown comprising an envelope 13 hermetically enclosing an electron gun 14 spaced from a fluorescent image display screen 17. The gun provides the source, control, acceleration and focusing of the electron beam 19, which is directed toward the screen 17 and deflected by the 31. The first electrostatic field or immersion lens 47 causes the electrons to leave cathode 27 and converge at the electron crossover position 45. As the electrons proceed through the immersion field 47, they enter into grid 37 through aperture 40 in a moderately divergent manner. Upon entering grid 37, the electrons in beam 19 are again converged by tri-potential lens 49 so that the 1 ,1 ends facing one another so that focusing field 49 extends back to end wall 39 of grid 37. In addition, it 'can be seen that the electrons in beam 19 converge toward and diverge from crossover point 45 at moderate angles. As far as it can be determined, the beam, when "properly focused',-has essentially a conical envelope or magnetic field coils 22 to produce an image display internal neck portion of the tube contacting snubbers 21 and the interior region of thebulb around lead. 23 may be coated with a layer of a graphite composition prior to aluminizing to insure electrical contact. p

The electrostatic focus electron gun structure 15 shown in Fig.2 is one form of gun structure of the present invention to be used in the cathode ray tube illustrated in Fig. 1. This structure, which will be referred to as a ring type mount, comprises an indirectly heated cathode sleeve 27 having electron emissive material 29 disposed on the end thereof. Spaced from the cathode in the direction of electron travel is a cylindrical first or control grid 31 formed to provide an end wall 33 having its central portion reduced in thickness adjacent the aperture 35 formed therein. The cylindrical second or screen grid 37, which is spaced from grid 31, also has an end wall 39 formed to provide an.aperture*40 aligned with aperture'35. The third grid 41 and fourth grid or anode 43 are formed as open-ended cylinders having internal diameters substantially equal to the diameter of grids 31 and 37. The ends of grids 41 and 43 facing each other are curved to prevent arc-overs since there is a large potential difference existing between these electrodes during tube operation. Snubbers 21 on the fourth grid 43 provide electrical contact between conductive coating 25 and the high voltage lead 23. These gun electrodes may be mounted relative toone another in the conventional manner by support pins or. straps connected to an insulating rod (not shown); Thus, it can be seen from Fig. 2 that the gun structure 15 com:

prises a cathode and four aligned serially arrayed, cy-.

mum beam diameter position or crossover point 45. This position appears to be located close to the control grid outline extending from the grid 37 vicinity to screen 17. with the apex of the cone located at the screen.

Formation of the electrostatic lens system comprising immersion field 47 and tri-potential focusing field 49 is dependent, in part, upon the selected object distance L the selected image distance L and upon the voltages selected and applied to the various electrodes or grids in gun 15. For purposes of description, the object distance L; may be defined as extending from crossover point 45 to the mid-point of the gap between grids 41 and 43,

which is essentially at the center 50 of lens 49. Since it has been found that the crossover is located close to grid 31, the distance L can also be considered to extend from the forward surface of this grid to the center of lens 49 for all practical purposes. The image distance L may be defined as the distance from the center of the lens 49 or the center of the grid 41-grid 43 gap to the center of screen 17.

It has been found that for a short axial length cathode ray tube 11, the ratio of object distance to image distance, i.e., L /L preferably is maintained within the range of from approximately .03 to .06. Above this range, the tube length is undesirably long and below this range the lens magnification is large and focusing becomes diificult with available receiver voltages. With a focusing lens system of this type, the object distance L can range from about .250" to .640" when using grid cyl' inder diameters of the type hereinafter described. It is to be understood that the L /L ratio and object distance range relates primarily to a deflection tube. Accordingly, if the deflection angle is increased, these parameters as well as other interrelated parameters to be hereafter described may have to be altered in accordance with the concept described herein. The image distance L; should be selected to be larger in tubes with a larger screen area than in tubes with a smaller screen area. For instance, when employing gun 15 with a tube using a 14 inch diagonally measured screen, the image distance may be about 8.5 inches while a tube with a 24 inch diagonally measured screen may require an image distance of 12 inches.

The voltages applied to the electrodes of gun 15 are also basic parameters inter-related with the object and image distances which determine the action of immersion lens 47 and tri-potential focusing lens 49 as well as the electron beam cut-off or drive range characteristics of the tube. A factor to be considered regarding the drive range is the desired beam current, e.g., 1000 microamperes, which is required to provide an acceptable highlight brightness on the screen. It is advantageous to minimize the drive requirements in order to minimize the drive requirements in order to minimize motion of the crossover point 45 and thus minimize beam spot bloom ing. When desired, the drive characteristics of gun 15 can be altered either by changing the spacing'between cathode 27 and grid 31, the spacing from grid 31 to grid 37, the diameter of aperture 35, the thickness of the material surrounding this aperture, or voltage V applied to grid 37.

Regarding the focusing system, the immersion field 47 is formed by the potential difference existing between V; on grid 31 and V; on grid 37 and V Assuming that tube 11 is cathode driven, although the inventive concent is not to be construed as being restricted thereto,

the cathode voltage V represents the video signal derived from the video amplifier or second detector in the receiver and applied to the'cathode. V mayrange from ground to approximately 50 volts positive. Under these conditions, V is usually held at ground potential and V is at a positive potential as will be hereinafter de-.

scribed. I

Tri-potential focusing lens 49 is formed by the rela tively low voltages- V and V and the relatively high voltage V-.,, applied to grids 37, 41 and 43 respectively; The potentials V and V preferably should neither exceed approximately 700 volts, nor be less than ground potential. At the upper part of this range, the focusing lens 49 becomes weaker and needs, for proper operation, a longer object distance and therefor a longer tube. In any event, V and V should preferably not exceed ap-. proximately 5% of the V or screen potential, which may range from about 12 kv. to 24 kv. as stated above.

The voltage V applied to screen grid 37 may be be, tween approximately 250 and 700 volts and preferably in the upper part of this range. The graph shown in Fig. 4 illustrates an advantage to selecting a voltage V: near the B boost potential. Using a constant cathode drive range, E of 50 volts, for example, the beam spot diameter at the screen is smaller at all beam currents when V is at 500 volts than at 300 volts. Stated generally, the beam spot diameter decreases, and thus image resolution increases, as V is increased, within limits.

The voltage V supplied to grid 41 preferably is less than V and in the vicinity of 300 volts since it has been found that there is a tendency for lens 49 to be less affected by line voltage fluctuations under these conditions. In practice, V may be a variable voltage ranging from ground to B boost as stated above. Varying V atfects the focusing of tri-potential lens 49 but does not noticeably affect the immersion field 47. Therefor, varying V does not appreciably change the beam cut-off or beam spot blooming characteristics of the tube.

The lengths of the various electrodes or cylinders in gun 15 determine in part some of the tube characteristics. Grid 31 is preferably long enough to substantially surround cathode 27, but otherwise as short as possible to minimize the length of mount 15. The total length of grids 37 and 41 taken together partially determine the object distance of lens 49. Individually, grid 37 may be slightly longer than grid 41. However, these lengths may be varied in order to stabilize the efiects of fields 47 and 49 under conditions of line voltage fluctuations. Proper selection of the form of and spacing between grids 37 and 41 in conjunction with selection of V and V provides minimum variations in focusing or beam spot size when there are proportional variations in the receiver supplied voltages V and V The length of grid 43 is not of primary importance but is preferably substantially as long as it is wide.

The spacings selected between electrodes in gun 15 also contribute to the desired tube characteristics. The cathode 27 to grid 31 spacing, and the grid 31 to grid 37 spacing are two of the several factors which determine the beam cutoff characteristics of the tube as mentioned above. With the voltages and dimensions enumerated for illustration of gun 15, the cathode to control grid 31 spacing may be from about .003 inch to .007 inch and the grid 31 to grid 37 spacing may be fromapproximately .010 inch to .025 inch.

The distance between grids 37 and 41 is interrelated with the lengths of these grids so that the proper object distance L; can be obtained. Thegrid 41 to grid 43 not to make the grid 41 to grid 43 distance sosmall that arc-overs occur between these electrodes.

The diameters of the electrodes in gun 15 are dictated, in part, by the prescribed :lens system. Generally,'larger diameter cylinders for grids 37, 41 and 43 produce a lens field 49 which has less spherical aberration and is desirable from this point of view. However, smaller diameter cylinders require a shorter object distance for particular operating voltages and therefore are more desirably adapted to shorter gun mount applications. Therefore, any selection of electrode diameters is the result of a compromise between spherical aberration and gun length. The grids, especially grid 41 in mount 15, preferably have an inside diameter within the range of from about .20 to .60 inch, but the invention is not to be construed as being restricted thereto. It is advisable to use a ratio of object distance L; to cylinder diameter D, i.e., L/D ranging from approximately 1.0 to 1.5. Since the object distance is dependent in part upon the available grid diameters, this object distance L may be shortened to an even greater extent if smaller diameter cylinders are em: ployed. Grid 43 may have a smaller diameter than the other grids to enhance focusing, if desired.

The physical configurations of grids 31, 37, 41 and 43 are selected to aid in the achievement of desired tube characteristics as well as for their low cost and adaptability to automatic production techniques. The end wall 33 of control grid 31 is reduced in thickness around aperture 35 in order to obtain the desired beam cut-off characteristics without necessitating anunusually small and impractical cathode 27 to grid 31 spacing or grid 31 to grid 37 spacing. This feature, however, is not essential; A thickness ranging from about .002 inch to .010 inch has proven satisfactory. Regarding aperture 35, if the diameter is too small, the cathode is overloaded and poor cathode emission life characteristics are experienced, and if the diameter is too large, the beam crossover area 45 becomes too large to obtain a reasonably small beam spot diameter at screen 17. It has been found that aperture 35 is preferably between about .015 inch and .030 inch for gun 15.

The apertured end plate 39 of screen grid 37 serves to separate focusing field 49 from immersion field 47. When using a control grid aperture 35 of the type described above, it has been found preferable to use a diameter for aperture 40 ranging from approximately .025 inch to .060 inch. If aperture 40 is too small the alignment of the grids becomes very critical, if too large it will allow the field lines of lens 49 to undesirably affect field 47.

Grids 41 and 43 are formed as open-ended cylinders and the end of grid 37 facing grid 41 is also open so that the field lines of lens 49 may have normal symmetry and extend back to the end plate 39.

The structural features of any picture tube gun are dependent to some extent upon the operating voltages, type of drive, beam cut-off limits, deflection angle, over all envelop length and the image definition and brightness characteristics desired. Therefore, the present invention may be altered in accordance with these desires without departing from the spirit and scope 'ofthe invention.

Without limiting the generality of the foregoing description of one embodiment of the invention, a specific example of gun mount 15, when used in a 17 inch degree deflection tube, may utilize an electrostatic focus ing system wherein the object distance L is about .426 inch and the image distance L is approximately 8.64 inches. ameters of the grids are approximately .375 inch.

With a cut-off drive range of about 60 volts,- the spacingbetween cathode 27 and. the internal, surface of grid 31 is about .005 inch, the aperture 35 diameter is about .022 inch, the wall thickness of this aperture is about .004 inch, and the spacing. between the. edge of aperture With the selected focus voltages, the inside di- 35 facing grid 37 and grid 37 is approximately .024 inch.

Aperture 40 in end wall 39 may have a diameter of about .036 inch to provide adequate separation of immersion field 47 from focusing field 49.

Acting in conjunction with the grid 31 to grid 37 spacing to provide the object distance recited above, grid 37 is about .165 inch long, grid 41 is about .135 inch long, the spacing between these grids is about .062 inch and the spacing from grid 41 to grid 43 is approximately .080 inch.

The length of grid 31 is about .250 inch and of grid 43 is approximately .345 inch.

In operation, gun 15 may have applied there a video signal V with V at ground, V at about 500 volts, V variable and at approximately 300 volts and V, at about 16 kv.

The invention disclosed above in conjunction with the ring type gun mount 15 may be used to produce other types of guns such as those illustrated in Figs. 5, 6, 7 and 8. The basic gun structure parameters as well as the lens and voltage characteristics herebefore described are applicable to these mounts.

Fig. illustrates a disc-ring type electrostatic tri-potential focus gun 51 employing a cathode 53 spaced from a cylindrical control grid 55 which has a limiting aperture 57 formed therein. A disc-shaped screen grid 59 is spaced from the control grid and is provided with an aperture 61. The open-ended cylindrical grids 63 and 65 are disposed in spaced relationship and arrayed toward the screen of the tube.

Electron gun 51 functions essentially in the same manner as gun 15 of Fig. 2. The immersion field 67 tends to form an electron crossover point and tri-potential field 69 operates to focus the beam at the screen. Voltages V V V V and V may be of the magnitude described above, with V preferably being variable.

Fig. 6 illustrates a disc-type electrostatic focus electron gun 71 employing in spaced array a cathode 73, cylindrical-control grid 75, disc screen grid 77, disc grid 79 and cylindrical grid 81. The immersion field 83 is formed between cathode 73 and grids 75 and 77 and a tri-potential focusing lens 85 is formed by the potentials applied to grids 77, 79 and 81 The inside diameter of disc 79 may be slightly smaller than grid 81 in order to confine the field of lens 85. Here also, the voltages applied to the various grids may have the order of magnitude as described above. a V

The ring type gun mount 90 illustrated in-Fig. 7 is modified from the gun 15 structure of Fig. 2 primarily in the cathode and control grid regions. In this embodiment of the invention, the cathode 89 and control grid 91 assembly is essentially constructed as a diode type receiving tube with an aperture 93 formed in member 91. The longitudinal axis of indirectly heated cathode 89 may be, in this instance, disposed transverse to the longitudinal axis of gun 90. Cylindrical grid 95 is formed with an end plate 97, which is provided with an aperture 99. That portion of grid 95 facing grid 91 is reduced in diameter to facilitate mounting to the previously fabricated cathode-control grid assembly. This reduced diameter does not appreciably affect the shape or action of the focusing lens. Serially arrayed in spaced relationship from grid 95- are the open-ended cylindrical grids 101 and 103. The immersion field 105 is formed between grids 91'and 95 and the tri-potential focusing field 107 is formed by the potentials applied to grids 95, 101 and 103. Voltages V V V V and V; are of magnitude described above. o

The several embodiments of the invention heretofor described are directed to an ultra-short tri-potential electrostatic'focus electron' gun employing fou'r' grids. In each instance, the net convergent-focusing field such" as lens 49 of Fig. 3 has a convergent portion within the region :of the second and third grids, e.g., electrodes 37 8 e.g., electrode 43. However, in accordance with this invention, it is to be understood that the number of grids used to form either the convergent or divergent portions of the focusing lens may be increased, if desired. For instance, Fig. 8 illustrates a gun structure 108 which employs four separately applied voltages in order to produce the focusing field 109. In this application, cathode 111 and control grid 113 are spaced from one another and from screen grid 115 to produce immersion field 117 when the voltages V V and V respectively are applied thereto. The convergent portion of lens 109 lies substantially within the axially aligned and spaced cylindrical grids 115, 118 and 119. The divergent portion of lens 109 lies substantially within cylindrical grid 121. This type of gun structure may be considered to use a tetra-potential focusing lens 109 since voltages V V and V all of which can be between ground and B boost, and the high potential V may be separately applied to the designated grids. An advantage of such a structure resides in its adaptation to a more refined control over the shape of field 109 since the voltages V V and V may be selected to arhieve this control after the V value is selected. In this respect, grid 121 may also be formed as two or more grids, with separate high voltages applied to each. Although increasing the number of cylinders or discs may improve control of the field shape of lens 109, the cost of parts and the need for available power supplied will also increase over the tri-potential lens structures. In order to appreciate the marked advance toward shorter axial length tubes which this invention provides, it is only necessary to compare the overall lengths of the shortest previously available tubes of a given type such as a 17 inch, 110 degree deflection tube, with a 17 inch 110 degree deflection tube employing one of the gun structures of this invention. It is to be noted that the dimensions fo the envelope or bulb in both instances from the yoke reference line forward to the screen are identical. The prior short tubes had an overall length from the base of the tube to screen 17 which was approximately 13 inches whereas the overall dimension of tube 11 is approximately 10 inches. The difference in these two lengths is in the neck portion of the tube and is brought about primarily by the ultra-short gun structures. v

A cathode ray tube constructed in accordance with the invention meets the operating requirements of tele} vision manufacturers and has satisfactory electrical and image brightness and definition characteristics. The focusing lens produces minimum spherical aberration and can be controlled without undesirably affecting the immersion field. Accordingly, with these features, the picture tube has universality in that the range of manufacturing tolerances need not be unusually restrictive and that the electron beam in the tube can be focused under all circumstances where the voltages normally available for application to' the focusing system electrodes are different from one television receiver to another.

Although several embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims. What is claimed is: 1.' A gun structure adapted for use in a cathode ray tube employing an electron beam comprising in sequence a cathode, a first grid having an apertured end wall, a second grid formed to provide an apertured wall, a third grid having a given diameter and a fourth high voltage grid, said grids being mounted in spaced relationship with one-another and formed to provide an electrostatic immersion lens and a tri-potential electrostatic beam focusing lens. separated directly by said secon'd'gridaper- .tured wall, saidfocusing lens having an 'object 'distance and 41, and a divergent portion-within the fourth=grid whichstis substantial but less'thanapproximately .64

inch, an object distance to image distance ratio ranging from approximately .03 to .06, and a ratio of said given object distance to said given diameter ranging from about 1.0 to 1.5, said tri-potential lens providing focusing of the beam when the voltage applied to said second grid ranges from approximately 250 to 700 volts, the voltage applied to said third grid is less than approximately 700 volts, and the voltage applied to said fourth grid exceeds approximately 12,000 volts.

2. A gun structure adapted for use in a cathode ray tube employing an electron beam comprising in sequence a cathode, a first grid having an apertured end wall, a second grid formed to provide an apertured wall, a cylindrical third grid, a cylindrical fourth grid having a given diameter, and a fifth high voltage grid, said grids being mounted in spaced relationship with one another and formed to provide an electrostatic immersion lens and a tetra-potential electrostatic beam focusing lens separated directly by said second grid apertured wall, said focusing lens having an object distance which is substantial but less than approximately .64 inch, an object distance to image distance ratio ranging from approximately .03 to .06, and a ratio of said given object distance to said given diameter ranging from about 1.0 to 1.5, said tetrapotential lens providing focusing of the beam when the voltage applied to said second grid ranges from approximately 250 to 700 volts, the voltages applied to said third grid and fourth grids are less than approximately 700 volts, and the voltage applied to said fifth grid exceeds approximately 12,000 volts.

3. A gun structure adapted for use in a cathode ray tube employing an electron beam including in sequence a cathode, a first grid having an apertured end wall, a second grid formed to provide an apertured wall, at least a third grid having a given diameter, and a final high voltage grid, said grids being mounted in spaced relationship with one another and formed to provide an electrostatic immersion lens and an electrostatic beam focusing lens separated directly by said second grid apertured wall, said focusing lens having an object distance which is substantial but less than approximately .64 inch, an object distance to image distance ratio ranging from approximately .03 to .06, and a ratio of said given object distance to said given diameter ranging from about 1.0 to 1.5, said focusing lens providing focusing of the beam when the voltage applied to said second grid ranges from approximately 250 to 700 volts, the voltage applied to said third grid is less than approximately 700 volts, and the voltage applied to said final grid exceeds approximately 12,000 volts.

References Cited in the file of this patent UNITED STATES PATENTS 2,139,678 Glass Dec. 13, 1938 2,227,033 Schlesinger Dec. 31, 1940 2,806,163 Benway Sept. 10, 1957 2,825,837 Dudley Mar. 4, 1958 2,839,703 Niklas June 17, 1958 2,859,366 Squier Nov. 4, 1958 OTHER REFERENCES Pearce: The Return of Electrostatic Focusing, Journal of the Television Society, vol. 8, No. 6, Apr.-June, 1957 PP- 237 to 248. 

